Compact wideband coplanar stripline bandpass filter with wide upper stopband and its application to antennas

A compact wideband coupled-coplanar stripline configuration is presented for a wideband bandpass filter design. Without any increase in the filter size, two stripline shunt open-stubs are introduced to create two transmission zeros to suppress the undesired third harmonic passband of the basic section. Also, a method to control the even- and odd-mode velocities of a coupled-coplanar stripline is described. A prototype bandpass filter having 3 dB fractional bandwidth of 100% at a centre frequency of 3.8 GHz is fabricated on a 0.813 mm thick RO4003 substrate with a dielectric constant of ε_r = 3.38 with an overall size of 14 × 4.3 mm². Moreover, a differential ultra-wideband (UWB) antenna is designed and the filter is applied in the co-design with the antenna. The resultant compact module provides good radiation characteristics over the 3-5 GHz UWB band and suppresses the other unwanted out-of-band signals.